The present invention relates to a composite electrode. The present invention also relates to a lithium-ion battery having such composite electrode, and a method for producing such composite electrode.
As used herein, the terms “lithium-ion battery”, “rechargeable lithium-ion battery”, and “secondary lithium-ion battery” are used synonymously. These terms also encompass the terms “lithium battery”, “lithium-ion accumulator”, and “lithium-ion cell”, and also all lithium or alloy batteries, including in particular lithium-sulfur, lithium-air or alloy systems. Therefore, the term “lithium-ion battery” is used as a collective term for the aforementioned terms which are customary known in the art. It refers to both rechargeable batteries (secondary batteries) and non-chargeable batteries (primary batteries). In particular, as used herein, a “battery” within the meaning of the present invention also encompasses an individual or single “electrochemical cell”.
Generally, as known in the art, the mode of action of a lithium-ion battery can be described as follows: the electrical energy is stored in lithium ions (at the negative electrode) and transition-metal oxides (at the positive electrode) in a chemical process with a change of material. Here, the lithium ions in the ionized form (Li+) can migrate back and forth between the two electrodes through an electrolyte, which contains usually lithium hexafluorophosphate (LiPF6) as the conducting lithium salt. In contrast to the lithium ions, the transition-metal ions present at the cathode are stationary.
This flow of lithium ions is necessary in order to compensate the external flow of electric current during charging and discharging, so that the electrodes themselves remain electrically neutral. During discharging, the effective lithium atoms (or the negative active material containing the lithium ions) at the negative electrode each release an electron, which flows via the external current circuit (load) to the positive electrode. At the same time, the same number of lithium ions migrates through the electrolyte from the negative electrode to the positive electrode. At the positive electrode, however, the lithium ions do not take up the electron again, but instead the transition-metal ions present there take up the electrons. Depending on the type of battery, these ions may be cobalt, nickel, manganese or iron ions, etc. The lithium thus continues to be in ionized form (Li+) at the positive electrode in the discharged state of the cell.
WO 2011/109815 A1 discloses composite electrodes having gradients of a series of chemical, physical and performance properties in the direction of the electrode thickness.
It is an object of the present invention to provide a composite electrode having improved ion conduction and high-current capability in addition to effect adhesion of an electrode composition to a collector.
This and other objects of the invention are achieved by means of a composite electrode in accordance with one or more aspects of the disclosure.
The following definitions apply, where applicable, to all aspects of the disclosure:
Lithium-Ion Battery
As used herein, the term “lithium-ion battery” has the meaning as defined above. In particular, the term also includes an individual or single “electrochemical cell”. Preferably, in a “battery”, two or more electrochemical cells of this kind are connected, either in series (that is, one after another) or in parallel.
Electrodes
The electrochemical cell of the invention has at least one positive electrode and at least one negative electrode, i.e., a cathode (positive electrode) and an anode (negative electrode).
These two electrodes each have at least one electrochemically active material. This material is capable of accepting or releasing lithium ions and at the same time takes up or releases electrons.
As used herein, the term “positive electrode” refers to the electrode which when the battery is connected to a load, such as to an electric motor, is capable of accepting electrons. In this nomenclature, it represents the cathode.
As used herein, the term “negative electrode” refers to the electrode which in operation is capable of releasing electrons. In this nomenclature, it represents the anode.
The electrodes include inorganic material or inorganic compounds or substances which can be used for or in or on an electrode or as an electrode. Under the operating conditions of the lithium-ion battery, on the basis of their chemical nature, these compounds or substances can take up (intercalate) lithium ions or metallic lithium and also release them. In the present description, a material of this kind is referred to as an “active cathode material” or “active anode material”, respectively, or generally, as “active material” or “active electrode material”. For use in an electrochemical cell or battery, this active material is preferably applied to a support, preferably to a metallic support, preferably using aluminum for the cathode and copper for the anode, respectively. This support is also referred to as a “collector” or a “current collector” or a “collector foil.”
Cathode (the Positive Electrode)
As for selecting the active material for the positive electrode (also referred to as the active cathode material), it is possible to use any active materials which are known in the art. These include, for example, LiCoO2 (LCO), lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), high-energy NCM (HE-NCM), lithium-iron phosphate, or Li-manganese spinel (LiMn2O4) or high-voltage spinel such as LiMn1.5Ni0.5O4. According to one aspect of the invention, any suitable active material known in the art can be used for the cathode (the positive electrode).
In one preferred embodiment, the active cathode material may be a material selected from the group consisting of a lithium transition-metal oxide (also referred to as the lithium metal oxide), layered oxides, spinels, olivine compounds, silicate compounds, and mixtures thereof. Such active cathode materials are described for example in Bo Xu et al. “Recent Progress in Cathode Materials Research for Advanced Lithium Ion Batteries”, Materials Science and Engineering R 73 (2012) 51-65. Preferably, the active cathode material is HE-NCM. Layered oxides and HE-NCM are also described in U.S. Pat. Nos. 6,677,082, 6,680,143, and 7,205,072 of the Argonne National Laboratory.
Examples of olivine compounds are lithium phosphates of empirical formula LiXPO4 where X═Mn, Fe, Co or Ni, or combinations thereof.
Examples of lithium transition-metal oxide, spinel compounds, and layered transition-metal oxides include lithium manganate, preferably LiMn2O4, lithium cobaltate, preferably LiCoO2, lithium nickelate, preferably LiNi0.8Co0.15Al0.05O2 (NCA), or mixtures of two or more of these oxides, or their mixed oxides thereof.
The active material may also contain mixtures of two or more of the substances described herein.
To increase the electrical conductivity, further compounds (such as conductivity additives) are included in the active material, preferably carbon-containing compounds, or carbon, preferably in the form of conductive carbon black or graphite. The carbon may also be introduced in the form of carbon nanotubes or graphene. Such additions are preferably in an amount of from 0.1 to 6 wt %, more preferably, from 1 to 3 wt %, based on the positive electrode's composition (excluding carrier solvent) applied to the support.
Anode (the Negative Electrode)
As for selecting the active material for the negative electrode (also referred to as the active anode material), it is possible to use any active materials which are known in the art. According to one aspect of the invention, any suitable active material known in the art can be used for the negative electrode (the anode).
The active anode material can be selected from the group consisting of lithium metal oxides, such as lithium titanium oxide, metal oxides (e.g., Fe2O3, ZnO, ZnFe2O4), carbon-containing materials, such as graphite (e.g., synthetic graphite, natural graphite), graphene, mesocarbon, doped carbon, hard carbon, soft carbon, mixtures of silicon and carbon, silicon, tin, metallic lithium and materials which can be alloyed with lithium, and mixtures thereof. It is also possible to use niobium pentoxide, tin alloys, titanium dioxide, tin dioxide, silicon or mixtures thereof as the active material for the anode (the negative electrode).
In one embodiment, the active anode material is a material which can be alloyed with lithium. This material may be metallic lithium, a lithium alloy, or an unlithiated or partially lithiated precursor thereof, from which a lithium alloy is produced on formation. Preferred materials which can be alloyed with lithium are lithium alloys selected from the group consisting of silicon-based, tin-based, and antimony-based alloys. Such alloys are described for example in the review article by W. J. Zhang, Journal of Power Sources, 196 (2011) 13-24.
Electrode Binder
The materials used for the positive or negative electrode, for example the active materials, are held together by one or more binders which hold these materials on the electrode and/or on the collector.
The binders can be selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), polyethylene oxide (PEO), polytetrafluoroethylene, polyacrylate, styrene-butadiene rubber, and carboxymethylcellulose (CMC), and mixtures and copolymers thereof. The styrene-butadiene rubber and optionally the carboxymethylcellulose and/or the further binders, such as PVdF, are preferably present in an amount of 0.5-10 wt %, based on the total amount of the active material used in the positive or the negative electrode.
Separator
The electrochemical cell of the invention has a material which separates the positive electrode and the negative electrode from one another. This material is permeable to lithium ions, i.e., conducts lithium ions, but is a nonconductor for electrons. Materials of this kind used in lithium-ion batteries are also referred to as separators.
In one preferred embodiment, polymers are used as separators. In one embodiment, the polymers are selected from the group consisting of: polyesters, preferably polyethylene terephthalate; polyolefin, preferably polyethylene, polypropylene; polyacrylonitrile; polyvinylidene fluoride; polyvinylidene-hexafluoropropylene; polyetherimide; polyimide, polyamide, polyethers; polyetherketone, or mixtures thereof. The separator has porosity, so that it is permeable to lithium ions. In one preferred embodiment, the separator consists of at least one polymer.
Electrolyte
As used herein, the term “electrolyte” refers to a liquid in which a conducting lithium salt such as lithium hexafluorophosphate (LiPF6) is in solution. The liquid is preferably a solvent for the conducting salt. In that case the conducting Li salt is preferably in dissociated form.
Preferably the solvents are chemically and electrochemically inert. Examples of suitable solvents include preferably organic solvents such as, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, sulfolanes, 2-methyltetrahydrofuran, or 1,3-dioxalane. Preferably, organic carbonates are used as the solvent.
In one aspect of the disclosure, ionic liquids can also be used as solvents. The ionic liquids contain exclusively ions. Examples of cations include those which can be in alkylated form, such as imidazolium, pyridinium, pyrrolidinium, guanidinium, uronium, thiuronium, piperidinium, morpholinium, sulfonium, ammonium, and phosphonium cations. Examples of anions which can be used include halide, tetrafluoroborate, trifluoroacetate, triflate, hexafluorophosphate, phosphinate, and tosylate anions.
Exemplary ionic liquids include the following: N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide, N-methyl-N-butylpyrollidinium bis(trifluoromethylsulfonyl)imide, N-butyl-N-trimethylammonium bis(tri-fluoromethylsulfonyl)imide, tri ethyl sulfonium bis(trifluoromethylsulfonyl)imide, and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide.
Preference is given to using two or more of the liquids described above. Preferred conducting salts are lithium salts which have inert anions and which are preferably nontoxic. Suitable lithium salts are preferably lithium hexafluorophosphate (LiPF6), or lithium tetrafluoroborate (LiBF4), and mixtures of one or more of these salts. In one embodiment, the separator here is wetted or impregnated with the lithium salt electrolyte.
Various objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying examples.
In one aspect of the disclosure, the present invention is directed to a composite electrode.
The composite electrode of the invention includes a collector, the collector is coated with an electrode composition which contains an active electrode material, a binder, and a conductivity additive. The electrode composition has a concentration gradient along the direction of the electrode thickness in respect of the active electrode material and the conductivity additive, with a concentration gradient of the active electrode material increasing in the collector direction, and a concentration gradient of the conductivity additive decreasing in the collector direction. The conductivity additive is preferably conductive carbon black.
The collector of the composite electrode of the invention may be coated with only one layer, within which the concentration gradients of the invention are formed. Alternatively, the concentration gradient of the invention may also be formed through a plurality of layers, each having a constant concentration. Moreover, both possibilities may also be linked—for instance, in that a layer having a concentration gradient in respect of at least one of the two gradients of the invention is connected to a plurality of layers having a constant concentration gradient.
The inventors have recognized that using the composite electrode of the invention, it is possible to achieve improved ion conduction and high-current capability in addition to effect adhesion of the electrode composition to the collector.
The binder concentration of the electrode composition in the direction of the electrode thickness is preferably constant.
In one preferred embodiment, the electrode composition, based on volume, possesses a porosity of 5% to 50%, and has a ratio of the porosity in a near-surface layer to the porosity of a layer near to the collector of 1.2 to 5, preferably at 1.5 to 4, more preferably at 1.5 to 2.5. The measurements in this context are determined by Mercury Porosimetry (or Hg porosimetry). A measurement technique of this kind is described for example in Van Brakel, J., S. Modrý, and M. Svata. “Mercury Porosimetry: State of the Art.” Powder Technology 29.1 (1981): 1-12.
In one preferred embodiment, the conductivity additive, preferably conductive carbon black, is present in the electrode composition in an amount of 1 to 7 wt % and the binder is present in the electrode composition in an amount of 1 to 7 wt %.
In one preferred embodiment, the electrode composition has a ratio of the weight-based amount of conductivity additive, preferably conductive carbon black, in a layer near to the collector to the weight-based amount of the conductivity additive, preferably conductive carbon black, of a near-surface layer of 1.2 to 5, preferably 1.5 to 4, more preferably 1.5 to 2.5.
In one preferred embodiment, the active electrode material is an anode material selected from the group consisting of synthetic graphite, natural graphite, carbon, lithium titanate, and mixtures thereof.
In another preferred embodiment, the active electrode material is a cathode material selected from the group consisting of lithium transition-metal oxide, layered oxides, spinels, olivine compounds, silicate compounds, high-energy NCM, and mixtures thereof.
In one preferred embodiment, the binder is selected from the group consisting of polyvinylidene fluoride, copolymer of polyvinylidene fluoride and hexafluoropropylene, copolymer of styrene and butadiene, cellulose, cellulose derivatives, and mixtures thereof.
The lithium diffusion coefficient of the active electrode material or of a mixture thereof at room temperature (20° C.) is preferably 1.0×10−4 cm2 s−1 to 1.0×10−14 cm2 s−1, more preferably, 1.0×10−7 cm2 s−1 to 1.0×10−14 cm2 s−1. The measurement values in this context are determined by the GITT (Galvanostatic Intermittent Titration Technique), as described for example in W. Weppner and R. A. Huggins, J. Electrochem. Soc., 124, 1569-1578 (1977).
Lithium diffusivity is an intrinsic property of an active electrode material. In the case of using a cathode as the composite electrode, it is possible to set a gradient of the lithium diffusion properties in the direction of the electrode thickness by selecting a suitable active cathode material or by selecting a mixture of two or more active cathode materials, independently of other gradients, such as in the porosity, for instance, by using a different active cathode material in the gradient direction and/or for each layer when using two or more layers. Another possibility is that of using a mixture of the same active cathode materials in each layer, but giving different weight to the proportions of the active cathode materials along the gradient and/or in each layer. These possibilities may also be combined. The same rules apply in the case of using an anode as the composite electrode. The gradient of the lithium diffusivity preferably decreases in the collector direction.
In one aspect, the particle size D50 of the secondary particles of the active electrode material is preferably from 0.05 μm to 50 μm, especially preferably from 2 μm to 30 μm, and/or the particle size of the primary particles of the active electrode material is preferably from 0.001 μm to 50 μm, especially preferably from 0.010 μm to 5 μm. The measurements in this context are determined by scanning electron microscopy (SEM). A measurement technique of this kind is described for example in U.S. Pat. No. 5,872,358.
The number of layers is preferably 1 to a maximum of 20, especially preferred is 3 to 10.
The thickness of the composite electrode without a collector is generally from 5 μm to 250 μm, preferably from 30 μm to 120 μm, more preferably from 40-80 μm. The measurements in this context are determined by optical methods, as specified in U.S. Pat. No. 4,008,523.
In another aspect of the disclosure, the present invention is directed to a lithium-ion battery comprising two electrodes, a separator, and an electrolyte, wherein at least one of the electrodes is a composite electrode according to the present invention.
In another aspect of the disclosure, the present invention is directed to two different methods for producing a composite electrode. Each of these two methods can be used to prepare the composite electrode according to the present invention.
In one embodiment of the invention, the method for producing a composite electrode having a collector, the collector is coated with an electrode composition comprising an active electrode material, a binder, and a conductivity additive, includes the steps of: a) combining at least the active electrode material, the binder in solution with a carrier solvent, and the conductivity additive, preferably conductive carbon black, to form a homogeneous slurry; b) applying the slurry to the collector; c) stripping off the carrier solvent under reduced pressure and/or at elevated temperature, to form a porosity in the slurry; d) adjusting the porosity by calendering, wherein steps a) to d) are repeated at least once, in the course of which in step a) the electrode composition is modified such that the electrode composition has a concentration gradient along the direction of the electrode thickness in respect of the active electrode material and the conductivity additive, with a concentration gradient of the active electrode material increases in the collector direction, and a concentration gradient of the conductivity additive decreases in the collector direction. In one embodiment, the composite electrode has layers of electrode materials that are deposited onto one another by tape casting and in this way the gradient according to the present invention is produced, and the layers are then joined to one another by means of heat and pressure.
In another embodiment of the invention, the method for producing a composite electrode having a collector, the collector is coated with an electrode composition comprising an active electrode material, a binder, and a conductivity additive, including the steps of: a) combining at least the active electrode material, the binder in solution with a solvent, and the conductivity additive, preferably conductive carbon black, to form a homogeneous slurry; b) applying the slurry to the collector; c) stripping off the solvent under reduced pressure and/or at elevated temperature, to form a porosity in the slurry; d) adjusting the porosity by calendering, wherein the electrode composition is modified in step b) by adjusting and utilizing the different densities of the electrode material by means of ascending or descending in the slurry, or by utilizing diffusion in the solvent, such that the electrode composition has a gradient concentration along the direction of the electrode thickness in respect of the active electrode material and the conductivity additive, with a concentration gradient of the active electrode material increases in the collector direction, and a concentration gradient of the conductivity additive decreases in the collector direction.